Hydrotalcite Catalyst Research Articles

Kornblum Oxidation Using Hydrotalcites as Heterogeneous Basic Catalysts

The oxidation of various benzyl halides 1 with DMSO took place in the presence of a hydrotalcite catalyst (40% w/w) to give the corresponding carbonyl compounds 2 with up to 89% yield. This catalyst was separated from reaction mixture by simple filtration, and reused four times in the oxidation of benzyl chloride with no significant loss of activity.

Application of molecular modelling in heterogeneous catalysis research

Traditionally, heterogeneous catalysis has largely been an experimental field. While this is still true today, it is increasingly recognized that computer modelling and simulation are important tools in the study and development of catalytic systems. Molecular modelling has the potential to provide new insights into reaction pathways, to predict properties of catalysts that have not yet been synthesized, and to bring information for a given system from many different experimental techniques into a coherent picture. A close interaction of modelling and experiment is vital. The most fruit-fully strategy for using modelling in catalysis research is likely to be a dual-feedback mode, where experiments (kinetic studies of reaction rates, thermodynamic information on adsorption, and spectroscopic data on molecular-level structure) are used to validate the modelling and modelling is used to explain experimental results, to suggest new experiments, and perhaps to substitute for experiments in the screening of different catalysts or reaction conditions. The objective of this work is to present the dual-feedback mode on three case studies from the branch of heterogeneous catalysis and to highlight some interesting outputs as a result of applying such approach. The first issued from the study of molecular structure effects on noble metal catalyst. It presents usefulness of such approach for probing the mechanism of surface processes in hydrogenation of CC unsaturated bonds. The second one demonstrates how could theoretical calculations help in characterizing of active site in cinchona modified platinum catalyst and also judged the mechanism theories in stereoselective hydrogenations. The last case study presents a possibility to predict properties of hydrotalcite catalyst used for aldolization of cyclic ketones before its synthesis and also to define its applicability.

Suzuki cross-coupling reactions over Pd(II)-hydrotalcite catalysts in water

We studied the Suzuki cross-coupling reaction between bromobenzene and phenylboronic acid at room temperature in aqueous media containing K 2CO 3 as the base and solids consisting of Pd(II) supported on an Mg/Al hydrotalcite as catalysts. Based on the results, a surfactant facilitating the dissolution of phenylboronic acid in water is required for the reaction to develop at room temperature. Sodium dodecylsulphate proved the most effective surfactant among those tested for this purpose, which included anionic, cationic and neutral compounds. The conversion and selectivity results were excellent and on a par with previously reported values. However, ICP-MS measurements of palladium content before and after the reaction revealed that some metal in the catalyst was dissolved, so the catalytic process was not completely heterogeneous; rather, the reaction involves substantial homogeneous catalysis.

Suzuki cross‐coupling reaction of aryl and heterocyclic bromides and aromatic polybromides on a Pd(II)‐hydrotalcite catalyst

AbstractThe Suzuki cross‐coupling reaction of various bromine‐containing substrates and phenylboronic acid in toluene at 90 °C on a Pd(AcO)2Py2 catalyst supported on an MgAl hydrotalcite, using K2CO3 as the base, was studied. The conversion and selectivity results obtained for many of the substrates were excellent and similar to those provided by more active or even homogeneous catalysts. The reactions of aryl polybromides and phenylboronic acid gave the corresponding polyaromatic compounds in variable yields depending on the particular substrate. Arylation occurred in a consecutive manner by substitution of the different Br atoms. ICP‐MS measurements of the palladium content of the catalyst performed prior to and after the reaction revealed that part of the metal is incorporated into the bulk solution; therefore, the catalytic process is not purely heterogeneous. Copyright © 2008 John Wiley & Sons, Ltd.

Toluene total oxidation over Co supported catalysts synthesised using “memory effect” of Mg–Al hydrotalcite

Abstract Cobalt supported on calcined hydrotalcite catalysts were prepared using the memory effect of Mg–Al hydrotalcite. Mg–Al was calcined at different temperatures (500, 600 and 700 °C) and dipped in a Co nitrate aqueous solution. Mg–Al was regenerated and Co catalysts were obtained after the calcination under air at 500 °C. The catalyst synthesised with hydrotalcite calcined at 700 °C (Co/Mg–Al(700)500) showed high activity and selectivity for toluene oxidation. This interesting catalytic behaviour was correlated to the high quantity of Co species reducible at low temperature.

Large-scale synthesis of carbon nanofibers on Ni–Fe–Al hydrotalcite derived catalysts

A series of Ni2+–Fe2+–Al3+ hydrotalcite (HT) catalysts with varying compositions have been prepared by coprecipitation from Fe2+. The structure and properties of the catalysts were carefully characterized by various methods such as XRD, EXAFS, TGA, TPR, N2 adsorption/desorption, XPS and H2 chemisorption. It was shown that pure HT structures can only be prepared at low Fe2+ concentrations, while mixed oxide phases with spinel structure will form simultaneously at high Fe2+ concentrations. Pure HT structures are very important for small particle size, narrow size distribution and large surface area of the resulting catalysts. XPS study demonstrated Fe enrichment on the catalyst surfaces, which might have partly contributed to the low hydrogen chemisorption capacity for the Fe-containing catalysts.

Efficient synthesis of benzoin methyl ether catalyzed by hydrotalcite containing cobalt

An efficient synthesis of benzoin methyl ether with benzaldehyde and methanol in the presence of MgCoAl–hydrotalcite catalyst has been developed. By this new method, not only the cyanide poisoning is avoided, but also the synthesis of benzoin methyl ether can be completed in one step instead of the traditional two steps with both condensation and etherification. A new way for major industrial processes to synthesize clean benzoin methyl ether was explored.

Transesterification of triglycerides in high and low quality oil feeds over an HT2 hydrotalcite catalyst

The use of a heterogeneous catalyst, in the transesterification reaction of refined and acidic cottonseed oil for the production of methyl-esters (biodiesel) has been studied. The basic Mg–Al–CO 3 hydrotalcite catalyst used showed a high activity for methanolysis and esterification reactions in a refined and an acidic cottonseed oil as well as in a representative high water content animal fat feed. The experiments were performed in a temperature range between 180 and 210 °C, in a batch reactor. The methanol to vegetable oil molar ratio was 6 to 1, while the catalyst concentration was fixed at 1 wt.% of the oil mass. Non-calcined and calcined forms of the catalyst were tested. The activity of the calcined catalyst was lower than the initial activity of the non-calcined catalytic system but it appeared the same with the reused non-calcined system.

Preparation of Au/MgxAlO hydrotalcite catalysts for CO oxidation

Abstract Gold catalysts of CO oxidation, supported on a solid base of MgxAlO (x = Mg/Al molar ratio) hydrotalcite, were prepared by a modified deposition precipitation method. The effect of various parameters that are involved in the preparation of catalysts were studied, including the pH and the concentrations of HAuCl4 in the initial solution, the Mg/Al molar ratio and the calcination temperature of the MgxAlO support as well as the calcination temperature of the Au/MgxAlO catalyst. The catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet–visible (UV–vis) and X-ray photoelectron spectroscopy (XPS) analyses. The pH and the HAuCl4 concentrations in the initial solution, and the Mg/Al molar ratio of MgxAlO affected the pH in the final solution, and determined the real gold loading in the resulting catalyst. The calcination temperatures of the MgxAlO support and the Au/MgxAlO catalyst dominated the ratio of gold states (Au3+/Au0) on the catalyst. The optimal catalyst, 2%Au/Mg2AlO(100), was obtained using the following preparation parameters: (1) 1 × 10−3 M HAuCl4, (2) pH 2 (without adjusting pH) in the initial solution, (3) Mg/Al = 2 (Mg2AlO) calcined at 100 °C as a support and (4) 2%Au/Mg2AlO catalyst calcined at 100 °C. This investigation confirms that not only gold loading of the catalyst is important, the ratio of gold states (Au3+/Au0) is also critical in determining the activity of the catalyst for CO oxidation.

Adsorption and reduction of nitrate in water on hydrotalcite-supported Pd-Cu catalyst

The hydrotalcite-supported Pd-Cu catalysts were successfully prepared by the impregnation or coprecipitation method, and their adsorption and catalytic reduction activity for nitrate in water were evaluated. The catalysts were characterized by X-ray diffraction (XRD) and surface area (BET) analysis. The results demonstrated that hydrotalcite-supported Pd-Cu catalysts could significantly adsorb nitrate ions, and then, effectively catalytically reduce them. The excellent adsorption ability for nitrate resulted from the regenerated layer structure of calcined hydrotalcite catalyst in nitrate aqueous solution. Nitrate was forced into the interlayer space and adsorbed on the external surface. The adsorption kinetics and the adsorption isotherm could be well described by pseudo-second-order model and the Langmuir model, respectively. The comparison of catalytic reduction with the adsorption for nitrate indicated that catalytic hydrogenation activity for nitrate increased with increasing adsorption capacity; nitrate reduction on hydrotalcite-supported Pd-Cu catalysts was a consecutive and dynamic adsorption and catalytic hydrogenation process. In addition, the catalyst obtained by coprecipitation method, with intact regeneration of hydrotalcite structure and a high dispersion of active metals, hold higher adsorption and catalytic activity than that prepared by co-impregnation method.

High activity of Mn-MgAl hydrotalcite in heterogeneously catalyzed liquid-phase selective oxidation of alkylaromatics to benzylic ketones with 1 atm of molecular oxygen

Mn-MgAl hydrotalcite, surface-enriched with Mn, was prepared by thermal decomposition of MgAl hydrotalcite followed by reconstruction of its structure in the presence of Mn(II) cations and atmospheric CO 2 as a guest inorganic anion through contacting the decomposed mass with aqueous Mn(II) nitrate solution. The Mn-MgAl hydrotalcite showed very high catalytic activity, stability, and reusability in the liquid-phase selective oxidation of a wide range of alkylaromatics to their corresponding benzylic ketones, using atmospheric pressure of molecular O 2 as a sole oxidant under solvent-free and mild reaction conditions. The catalytic performance of Mn-MgAl hydrotalcite was found to be much higher than that of Mn-containing hydrotalcites prepared by different methods or MgAl hydrotalcite catalysts containing various other transition metals and synthesized through similar procedures.

Catalytic NO–H2–O2 reaction over Pt/Mg–Al–O prepared from PtCl62−- and Pt(NO2)42−-exchanged hydrotalcites

The catalytic NO–H2–O2 reaction at low temperatures (50–130°C) has been investigated over Pt-supported Mg–Al binary oxides, which were prepared from PtCl62− and Pt(NO2)42−–exchanged hydrotalcite(HT)-like compounds by heating at 600°C in H2. Although the NOx conversion of both catalysts was similarly high (>80%) at around 70°C, the Pt(NO2)42−–HT catalyst exhibited the higher selectivity to N2 (53% N2 and 47% N2O), compared to 12% N2 and 88% N2O for the PtCl62−–HT catalyst. The XANES/EXAFS spectroscopy showed that Pt prepared from PtCl62−–HT is partly oxidized due to the coordination of residual chlorides, whereas Pt(NO2)42−–HT yielded highly dispersed metallic Pt. Because of the presence of chloride strongly bound to Pt, the oxidative NO adsorption as NO2/NO3 was inhibited for PtCl62−–HT. The improved N2 selectivity with an increase of oxidative NO adsorption is in accord with the catalytic property of Na–Pt/ZSM-5 in our previous work [M. Machida, T. Watanabe, Appl. Catal. B: Environ. 52 (2004) 281.], suggesting that the N2 would be formed via NO2/NO3 intermediates.

Efficient N-arylation of amines catalyzed by Cu–Fe–hydrotalcite

An efficient N-arylation of amines with aryl halides in presence of heterogeneous recyclable Cu–Fe–hydrotalcite catalyst has been developed.

Novel Chiral Sulfonato‐Salen‐Manganese(III)‐Pillared Hydrotalcite Catalysts for the Asymmetric Epoxidation of Styrenes and Cyclic Alkenes.

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A hydrotalcite-based catalyst system for the single-stage liquid-phase synthesis of MIBK

A comparative study was performed between Pd supported on activated hydrotalcites bi-functional catalysts and physical mixtures of activated hydrotalcites with Pd supported on carbon nanofibers for the single-stage liquid-phase synthesis of methyl isobutyl ketone from acetone and H 2. Since it was found earlier that the dehydration reaction of diacetone alcohol over activated hydrotalcites is the rate-determining step in this process, Mg–Al hydrotalcites were investigated in the condensation of acetone to diacetone alcohol and its dehydration to mesityl oxide to enhance the activity. A correlation between the number of active base sites, as determined by volumetric CO 2 adsorption measurements at low pressures, and the activity in the dehydration reaction could be derived. A dependency on the Pd-loading (0–1 wt%) of Pd supported on activated hydrotalcites catalysts was found in the single-stage synthesis of methyl isobutyl ketone, whereas no such an effect was found when using physical mixtures of activated hydrotalcites with Pd on carbon nanofibers. Results from TEM, CO 2 chemisorption and N 2 physisorption as well as catalytic experiments in the hydrogenation of mesityl oxide and of cyclohexene indicate that this dependency is largely accounted for by entrapment of Pd particles in the agglomerates of the irregular stacks of hydrotalcite platelets, making these sites inaccessible for the reactants.

Improved Fe/Mg-Al hydrotalcite catalyst for Baeyer–Villiger oxidation of ketones with molecular oxygen and benzaldehyde

Metal (Me: Fe, Co, Ni or Cu) containing Mg-Al hydrotalcite-type anionic clay catalysts have been prepared by adopting the “memory effect” of hydrotalcite and tested for Baeyer–Villiger oxidation of ketones. Mg-Al hydrotalcite was calcined to the mixed oxide and dipped in an aqueous solution of metal (Me) salt; metal species was incorporated by “memory effect” to form Me/Mg-Al hydrotalcite as the active phase on the surface of the Mg-Al mixed oxide. The activity and the structure of these catalysts have been compared with those of metal supported catalysts prepared by coprecipitation and impregnation. Among the metals tested, iron was the most effective and use of Fe(NH 4) 2(SO 4) 2·6H 2O as the metal salt resulted in the highest activity. The activity of the catalyst was higher than those prepared by coprecipitation from the nitrates of Mg, Fe and Al. Judging from Mössbauer and Fe K-edge XAFS spectra, Fe species possess the Fe 3+ valence state, are mainly octahedrally coordinated and form Fe O Fe cluster-type structure on the Mg-Al mixed oxides. This clearly indicates that the Fe 3+ O Fe 3+ cluster-type compounds are more active for the Baeyer–Villiger oxidation than well dispersed Fe 3+ species formed on the hydrotalcite prepared by coprecipitation. Six-membered cyclic ketones, such as cyclohexanone, 2-norbornone and 2-adamantanone, were effectively oxidized to the corresponding lactones by using O 2/benzaldehyde as an oxidizing agent.

On the Nature and Accessibility of the Brønsted-Base Sites in Activated Hydrotalcite Catalysts

Base catalysis is of importance for organic synthesis in general and fine chemicals manufacture in particular. Activated hydrotalcites have recently received a great deal of attention as solid base catalysts; however, no systematic work on the nature of their active sites has been published up till now. In this work two different methods have been applied to activate Mg-Al hydrotalcites to obtain Brønsted-base catalysts for liquid-phase condensation reactions. Activation via thermal treatment followed by rehydration (HT-reh) resulted in irregularly stacked platelets ( approximately 60 nm), whereas the sample activated via aqueous ion-exchange (HT-exc) preserved its original hexagonal hydrotalcite platelets ( approximately 100 nm). The specific activity for the self-condensation of acetone of HT-reh was over 10 times that of HT-exc. The enthalpy of CO2 adsorption on the activated hydrotalcites determined with calorimetry to gain insight into the strength of the basic sites showed very similar values. IR spectra of adsorbed CDCl3 as probe molecule on the differently activated samples revealed large differences in adsorbed amounts, but again the strength of the basic sites appeared to be the same. These results point to steric hindrance for the substrate molecules as the main factor determining differences in catalytic activity. The high accessibility of Brønsted-base sites in HT-reh is proposed to involve a distorted edge structure of the platelets. The edge structure of exchanged samples could be distorted too, either by exchange under reflux conditions or under ultrasonic treatment. In line with the proposed model, the distorted exchanged samples displayed a much higher catalytic activity than HT-exc.

Transesterification of Soybean Oil to Biodiesel by Using Heterogeneous Basic Catalysts

Biodiesel production has increased greatly in recent years, because of the less-detrimental effects of this fuel on the environment, compared to a conventional diesel obtained from petroleum. This work investigates the possibility of using MgO and calcined hydrotalcites as catalysts for the transesterification of soybean oil with methanol. The achieved experimental data show a correlation not only with the catalysts basicity, but also with its structural texture. However, the structural texture of the examined catalysts is dependent on both the precursor and the preparation method. At least four different types of basic sites have been individuated on the surface of MgO and calcined hydrotalcite catalysts. The strongest basic sites (super-basic) promote the transesterification reaction also at very low temperature (100 °C), while the basic sites of medium strength require higher temperatures to promote the same reaction. Ultimately, all the tested catalysts are resistant to the presence of moisture in the...

New Peroxo-polyoxometalates Pillared Hydrotalcite Catalyst for Propylene Epoxidation by Molecular Oxygen in Methanol

Abstract Peroxo-polyoxometalates pillared hydrotalcite has been synthesized by decomposing [PW11O39]7− pillared MgPdAl-type hydrotalcite with 30% H2O2; the novel heterogeneous catalyst showed 91.5% selectivity for propylene oxide at 47.5% propylene conversion for propylene epoxidation by O2 in methanol at 353 K for 10 h.

An efficient synthesis of 5-substituted 1 H-tetrazoles using Zn/Al hydrotalcite catalyst

Zn/Al hydrotalcite is an effective heterogeneous catalyst for the (2 + 3) cycloaddition of sodium azide with nitriles to afford 5-substituted 1 H-tetrazoles in good yields. The catalyst is recovered and reused for several cycles with consistent activity.